1. Field of the Invention
This invention relates generally to threaded tubular joints or connections and to seals for such connections. More specifically, the invention relates to a tubular joint for connecting the male or pin end of a pipe member to the female or box end of a pipe member in which cylindrical surfaces on the box and pin engage to provide a metal-to-metal seal for the connection.
2. Description of the Prior Art
The use of tubular threaded connections for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid under pressure is well known. Oil field tubular goods, for example, use such threaded connections for connecting the adjoining sections of conduit or pipe. Oil and gas wells currently are being drilled which extend for thousands of feet into the surrounding subterranean formations. In the drilling of such wells, tool joints having oppositely extending threaded couplings are used to form a drill string and support a drill bit at a lower end thereof. Once the well has been drilled to the final desired depth, a casing having a diameter greater than the diameter of the drill pipe is driven into the earth. The casing surrounds and supports tubing and prevents the sidewalls of the borehole from collapsing. In this discussion, the xe2x80x9cpipe memberxe2x80x9d will be understood to refer to tubing, casing, production pipe, drill pipe, special offshore platform tubulars, construction industry horizontal directional drilling tubulars, etc.
The connections for strings of drill pipe, tubing or casing must be able to withstand the total weight of a string of pipe many thousands of feet long. Since the drill string must also be used for the purpose of drilling, the joints must be able to withstand high torque loads, as well. Additionally, wells may not be driven in exactly vertical fashion or even in straight line fashion. Horizontal drilling operations are common today. The tubing used to drill the well and/or convey fluid from the well must be able to follow the course of the well as greater depths are reached. This places bending loads on the drill, casing or tubing strings at various places along the length thereof. Sections of tubing or casing are sometimes driven into the well, resulting in compressive loads being exerted. Thus, in addition to withstanding tremendous tensile loads, the threaded connections in the pipe string must also be able to absorb considerable compression loads.
The pipe strings of the type under consideration must have joints that provide a seal against leakage between mating threaded members. This can be achieved by providing a metal-to-metal seal, upon make-up. It is important that the mating sealing sections be free of defects or damage because, unless substantial surface-to-surface contact is maintained, leakage will likely occur. This is particularly true in the case of very deep wells due to the extreme fluid pressures involved.
Damage also can occur during the assembling of the joints in that the components to be assembled are heavy and unwieldy. Thus, it is often too difficult to handle the pipe sections without damage, particularly as a new pipe section is being xe2x80x9cstabbedxe2x80x9d into a coupling. The result is that threads can be blemished and that sealing surfaces can be damaged.
It is typically desirable to have a positive stop for the connection so that when a pre-determined level of threaded engagement has been reached, rotation of the pipe section will be stopped abruptly. This is necessary to accurately control the torque applied to the pipe section so that portions of the joint are not over stressed. The necessity of a positive stop has, on occasion, limited the available designs which could be utilized for the threaded connections.
Known prior art threaded connections used on oil field tubular goods often utilized a combination of specially designed xe2x80x9cpremiumxe2x80x9d threads and tapered (conical) sealing surfaces that engage to form a metal-to-metal seal to contain high pressures. The premium threads often generate radial interference as a means for retaining the make-up torque of the connection and also to provide a resistant seal. In some designs, a resilient seal ring was also included.
In order to contain high pressures, the radial interference of the metal-to-metal conical sealing surfaces must be significantly greater than that of the threads to generate sufficient bearing stress to overcome the effect of machining tolerances and taper on the height of the threads most adjacent the seal and the effects of excessive pipe compound trapped in the threaded area. The presence of excessive metal-to-metal contact bearing stress at the conical sealing surfaces of the prior art containment seals often resulted in make-up damage, i.e., galling with the result that the sealing mechanism did not function reliably. More importantly, conical seals have a desirable nominal axial makeup position. Non-shouldered premium connections have critical diametrical tolerances and makeup torque restrictions to guarantee an axial makeup position so the conical seal does not accumulate excess or deficient diametrical interferences.
Known prior art designs also exist that combine a conical metal-to-metal seal with a wedge thread as a torque stop. These type designs typically required a shallower angle for the metal-to-metal seal because of lenient axial tolerances required to machine the threads. In order to generate enough radial interference in the metal-to-metal seal to effect an efficient sealing mechanism, a great deal of rotation was required between initial contact of the sealing regions to final make-up. The longer the metal-to-metal seals were in contact during such rotation, the greater the tendency for galling. On the other hand, if interference was decreased to lessen the amount of rotational contact, sufficient contact forces were not always present to effect a reliable seal at final make-up.
Certain of the prior art seals described above were adequate for applications where liquid was being passed through the conduit. However, in applications where a gas was passed through the conduit, additional metal-to-metal seal regions were required.
The present invention has as one object to provide a threaded pipe connection having a cylindrical metal-to-metal, high pressure containment seal, or multiple seals, which exceed the capabilities of presently available tapered or conical metal-to-metal containment seals.
Another object of the invention is to provide such a cylindrical seal in a threaded pipe connection in which the mechanics of the seal have separated components that function independently, thereby having a unique function and purpose for each of the separate components.
Another object of the invention is to provide a threaded pipe connection with cylindrical metal-to-metal sealing surfaces rather than conical or tapered seal surfaces and with separated, tapered ramp regions which simulate pin swaging and/or box expansion, resulting in the ability of the cylindrical components of the seal to assemble without interference or with only very slight interference.
Another object of the invention is to provide a reliable metal-to-metal seal for a non-lubricated capable connection of the type commonly referred to in the industry as a dope-less connection.
The threaded pipe connection of the invention is characterized as having a cylindrical metal-to-metal, high-pressure containment seal capable of sealing both liquids and gases. The connection includes a first pipe member having a box end, the box end having an end opening defining an interior surface with internal threads. The internal threads are defined by crests and roots and opposing flanks. A second, mating pipe member has a pin end. The pin end has an exterior surface with mating external threads. The external threads also have crests and roots and opposing flanks, at least selected ones of which move into engagement with the internal threads of the box when the connection is made up. A generally cylindrical sealing surface on the box interior surface and a mating generally cylindrical sealing surface on the pin exterior surface form a primary containment seal upon make-up of the connection. A primary ramp region is formed on the pin exterior surface and a mating primary ramp region is formed on the box interior surface. The primary ramp regions are spaced apart from the cylindrical sealing surfaces which form the primary containment seal. The primary ramp regions are positioned with respect to the cylindrical sealing surfaces to simulate pin swaging and/or box expansion prior to full engagement of the cylindrical sealing surfaces, whereby the cylindrical sealing surfaces assemble with little or no interference or clearance.
Preferably, the primary ramp regions are selectively positioned to contact and interfere prior to full engagement of the cylindrical sealing surfaces and to be in full or near clearance as the full engagement of the cylindrical sealing surfaces occurs during the make-up of the connection. The mating cylindrical surfaces can be positioned on the box interior surface and/or on the pin exterior surface, respectively, so as to provide a desired degree of overlap before receiving an amount of interference necessary to form the primary containment seal for sealing off high-pressure liquids and/or gases. The threaded connection can also be provided with mating multiple ramp regions to transfer a selected gradual degree or radical degree of diametrical interference to the cylindrical sealing surfaces.
In a preferred form of the invention, the primary ramp regions are positioned on the box interior surface and on the pin exterior surface in preselected locations, whereby the pin end ramp region first rides up and onto the box primary ramp region as the connection is made up, whereby engagement of the primary ramp regions exerts an inward radial force on the pin end of the second, mating pipe member. The pin end primary ramp region then rides down the box primary ramp region as the connection continues to be made up. The primary ramp regions then enter a clearance region as the cylindrical sealing surfaces make contact and move to a fully engaged position.
Additional objects, features and advantages will be apparent in the written description which follows.